An oncornavirus-like particle has been purified from human leukemic spleens. The particle has a density of 1.16g/cc, a sedimentation coefficient of 560S, RNA dependent DNA polymerase activity, 70S RNA and the morphologic characteristics on electron microscopy of an immature C-type particle. This particle could not be purified from nonleukemic human spleens. BIBLIOGRAPHIC REFERENCES: Spiegelman, S., Axel, R., Baxt, W., Kufe, D., Schlom, J. The molecular genetics of human cancer and its etiologic implications. Symposium on Oncogenetics: XIII International Congress of Genetics. Genetics 79:317-338, 1975. Baxt, W. The role of viruses in inducing neoplasia in blood cells with special reference to the isolation of putative human leukemia virus. In The Leukemia Cell, ed. A.D. Rubin, 1976, in press.